CASSANO ALL' JONIO, Italy, June 21 (Reuters) - Pope Francis, visiting a stronghold of one of Italy's most dangerous crime groups, on Saturday comforted the jailed father of a three-year-old boy killed in an ambush and condemned mob violence against children.

Francis flew by helicopter to the town of Cassano All' Jonio in the southern Calabria region, home of the mafia-style 'Ndrangheta which investigators say has spread around the world.

The pope made the trip in part to pay tribute Nicola "Coco" Campolongo, who was killed in the town along with his grandfather in an organised crime attack last January.

The charred body of the boy, who had been entrusted to his grandfather Giuseppe Iannicelli after his parents were jailed on drugs charges, were found along with those of Iannicelli and a Moroccan woman in a burnt-out car in the town.

Francis, who last January strongly denounced the murder and asked the killers to repent, comforted the boy's father and other relatives during a meeting a Vatican spokesman called highly emotional.

"Never again violence against children. May a child never again have to suffer like this. I pray for him continuously. Do not despair," the spokesman quoted the pope as saying.

The boy's parents and grandfather were part of a drugs trafficking clan of the 'Ndrangheta. Social workers have come under criticism for entrusting the boy to his maternal grandfather, a convicted drugs runner who was out on bail.

STAND AGAINST ORGANISED CRIME

The crime group has been much harder for investigators to combat than the Sicilian Mafia because its structure is more lateral than hierarchical and its tightly-knit crime families are less flashy than the Sicilian mob and harder to penetrate.

A 2013 study by Demoskopia, an economic and social research institute, estimated the 'Ndrangheta's annual turnover at some 53 billion euros ($72 billion) in 30 countries, equivalent to about 3.5 percent of Italy's total official economic output.

Around half of its revenues came through drug trafficking, the study found.

In the prison, which holds a number of 'Ndrangheta criminals, the pope told the inmates they should not waste their time behind bars but seek forgiveness from God for their crimes and emerge rehabilitated.

Francis, who has condemned organised crime several times since his election in March, 2013, later addressed priests in the cathedral of Cassano all' Jonio, a run-down town of mostly drab concrete houses in the mountains near the Adriatic.

The bishop of Cassano, Nunzio Galantino is seen as one of the most progressive in Italy's poorer, underdeveloped south and has taken strong stands against organised crime.

But there have been instances of collusion of some priests in other areas of Calabria where the 'Ndrangheta is strongest, further south along the Italian peninsula near Reggio Calabria. ($1 = 0.7366 Euros)

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